In mathematics, the Zak transform (also known as the Gelfand mapping) is a certain operation which takes as input a function of one variable and produces as output a function of two variables. The output function is called the Zak transform of the input function. The transform is defined as an infinite series in which each term is a product of a dilation of a translation by an integer of the function and an exponential function. In applications of Zak transform to signal processing the input function represents a signal and the transform will be a mixed time–frequency representation of the signal. The signal may be real valued or complex-valued, defined on a continuous set (for example, the real numbers) or a discrete set (for example, the integers or a finite subset of integers). The Zak transform is a generalization of the discrete Fourier transform. The Zak transform had been discovered by several people in different fields and was called by different names. It was called the "Gelfand mapping" because Israel Gelfand introduced it in his work on eigenfunction expansions. The transform was rediscovered independently by Joshua Zak in 1967 who called it the "k-q representation". There seems to be a general consensus among experts in the field to call it the Zak transform, since Zak was the first to systematically study that transform in a more general setting and recognize its usefulness.
Continuous-time Zak transform: Definition In defining the continuous-time Zak transform, the input function is a function of a real variable. So, let f(t) be a function of a real variable t. The continuous-time Zak transform of f(t) is a function of two real variables one of which is t. The other variable may be denoted by w. The continuous-time Zak transform has been defined variously.
Definition 1 Let a be a positive constant. The Zak transform of f(t), denoted by Za[f], is a function of t and w defined by
Z a [ f ] ( t , w ) = a ∑ k = − ∞ ∞ f ( a t + a k ) e − 2 π k w i {\displaystyle Z_{a}[f](t,w)={\sqrt {a}}\sum _{k=-\infty }^{\infty }f(at+ak)e^{-2\pi kwi}} .
Definition 2 The special case of Definition 1 obtained by taking a = 1 is sometimes taken as the definition of the Zak transform. In this special case, the Zak transform of f(t) is denoted by Z[f].
Z [ f ] ( t , w ) = ∑ k = − ∞ ∞ f ( t + k ) e − 2 π k w i {\displaystyle Z[f](t,w)=\sum _{k=-\infty }^{\infty }f(t+k)e^{-2\pi kwi}} .
Definition 3 The notation Z[f] is used to denote another form of the Zak transform. In this form, the Zak transform of f(t) is defined as follows:
Z [ f ] ( t , ν ) = ∑ k = − ∞ ∞ f ( t + k ) e − k ν i {\displaystyle Z[f](t,\nu )=\sum _{k=-\infty }^{\infty }f(t+k)e^{-k\nu i}} .
Definition 4 Let T be a positive constant. The Zak transform of f(t), denoted by ZT[f], is a function of t and w defined by
Z T [ f ] ( t , w ) = T ∑ k = − ∞ ∞ f ( t + k T ) e − 2 π k w T i {\displaystyle Z_{T}[f](t,w)={\sqrt {T}}\sum _{k=-\infty }^{\infty }f(t+kT)e^{-2\pi kwTi}} . Here t and w are assumed to satisfy the conditions 0 ≤ t ≤ T and 0 ≤ w ≤ 1/T.
Example The Zak transform of the function
ϕ ( t ) = { 1 , 0 ≤ t < 1 0 , otherwise {\displaystyle \phi (t)={\begin{cases}1,&0\leq t<1\\0,&{\text{otherwise}}\end{cases}}}
is given by
Z [ ϕ ] ( t , w ) = e − 2 π ⌈ − t ⌉ w i {\displaystyle Z[\phi ](t,w)=e^{-2\pi \lceil -t\rceil wi}}
where ⌈ − t ⌉ {\displaystyle \lceil -t\rceil } denotes the smallest integer not less than − t {\displaystyle -t} (the ceiling function).
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